Crawlspaces under buildings have a natural tendency to be damp or wet. These conditions may often lead to a number of undesired problems. These problems may include premature failure of house main support beams, premature failure of floor joists; failure of floor joist insulation as a result of water saturation; rapid mold growth (and since most homes have some portion, if not most of the HVA system in the crawlspace, the mold has an opportunity to become airborne in the home); insect and rodent infestation; and severe cupping and warping of hardwood flooring.
Conventionally, crawlspaces are vented. The premise of a vented crawlspace is that outside air will circulate in the crawlspace and have a drying effect. Because a crawlspace is subject to outdoor temperatures, insulation is typically installed in the floor joist not only to insulate the home, but also to help protect exposed plumbing. Conventionally, a layer of polyethylene film or other plastic (collectively “poly” or “poly film”) is laid on the floor of the crawlspace. While local codes may vary, the poly coverage typically can range from 80 percent to 90 percent, but is rarely if at all 100 percent.
Large amounts of water and/or moisture may accumulate in conventional vented crawlspaces for at least two reasons: varying outdoor conditions and ground water. When outdoor conditions are moderate, mild temperatures with low humidity, drying can in fact occur in a crawlspace. The problem comes when the ambient air is hot with moderate to high humidity levels. When this hot/humid air circulates into a crawlspace, where it is considerably cooler, the moisture in the “fresh” air will condense. Just as dew can be seen on grass in the mornings, it is possible to see dew in a crawlspace. In extreme cases, the dew may actually drip and run from saturated floor joists and insulation. With respect to ground water, even though drain tile may be installed around a crawlspace perimeter, it is still possible for moisture from the ground to evaporate into the crawlspace, because the poly film on the ground may have numerous gaps in it. In many cases, water may actually be running onto the floor of the crawlspace as a result of the dew collected on the floor joist, insulation and main beams.
A more modern approach to crawlspace management is referred to as a “conditioned” crawlspace. The premise of a conditioned crawlspace is that the air that circulates in the crawlspace will come from the “conditioned” (heated and cooled) air inside the home. Since the crawlspace is not exposed to outdoor conditions (no exterior vents in a conditioned crawl) and it is both heated and cooled to some degree, it is necessary to insulate the exterior walls of the foundation instead of between of the floor joist. The floor of the crawlspace must be covered with poly film substantially 100 percent. In order to be effective, the plastic must completely seal the crawlspace from any ground water. While utilizing a conditioned crawlspace has some ancillary benefits such as energy savings, it is clear that the main purpose of a conditioned crawlspace is to directly address the moisture and resulting problems inherent to a vented crawlspace.
While conditioned crawlspaces are excellent in theory, there is at least one major obstacle that prevents the overwhelming use of the system—it is practically impossible to achieve a 100 percent seal of the crawlspace using products presently available. While it is relatively easy to seal a floor plastic at the flat portions of the wall of a crawlspace, sealing the plastic at a building pier presents difficulty. The current practice is to cut the plastic sheets of flooring to precisely fit a pier. Some type of sealer is then used to achieve, as close as possible, a 100 percent seal. While this is hypothetically effective, it is practically difficult to achieve in the field. A comparison of doing this would be to gift wrap a package so that the comers and seams were all perfectly aligned and tight, the only catch being that you would have to make all the cuts and folds before placing a box inside the wrapping.